The enzyme-linked immunosorbent assay (ELISA) is the main method used in medical inspections, such as those for food safety or cancer screening. Conventional ELISA technology has high sensitivity, but it also has other shortcomings such as ELISA kits being expensive, time-consuming to use, and requiring complicated inspection steps. Moreover, the analyte should be tagged by a bio-tag (such as fluorescent molecules) or enzyme for a color reaction, but the fluorescent molecules interfere with the actions of target molecules. Most of the fluorescent molecules have problems such as photobleaching or blinking, so that ELISA errors occur easily when detecting an analyte of low concentration.
Chips of localized surface plasmon resonance (LSPR) can be free of any labeling with a fast inspection effect, in which the surface plasmon resonance spectra of a metal nano structure are utilized. Because the LSPR is sensitive to a refractive index change of a metal-dielectric interface, the chip can be used to detect a trace amount of an analyte (e.g. antigen, antibody, DNA or cell) absorbed on a surface of the nano structure with a distance of tens of nanometers therebetween. In an LSPR chip, the change of the surface plasmon resonance spectra of the metal nano structure to the surrounding refractive index variation (Δλ/Δn) is critical for detection sensitivity. In addition, the narrower full width at half maximum (FWHW) of the resonance spectra means a higher spectra resolution and a better detection sensitivity. The figure of merit (FOM) of the chip can be defined as Formula 1:
                              Figure          ⁢                                          ⁢          of          ⁢                                          ⁢          Merit          ⁢                                          ⁢                      (            FoM            )                          =                                                            Δ                ⁢                                                                  ⁢                λ                                            Δ                ⁢                                                                  ⁢                n                                      ⁢                          (                              nm                /                RIU                            )                                            FWHM            ⁡                          (              nm              )                                                          (                  Formula          ⁢                                          ⁢          1                )            
Compared to conventional ELISA technology, the LSPR technology has several advantages such as being free of fluorescence labeling, reducing inspection steps and time, and no steric effect of conjugated secondary antibody (containing fluorescent molecules). However, the LSPR chip has a lower sensitivity than the ELISA method due to a wider FWHW of the general localized plasmon resonance spectrum. In addition, whether or not the analyte is close to hot spots (effective sensing area) of the nano structure is also critical for producing an effective spectra shift.
Accordingly, a novel LSPR structure is called for to improve the detection sensitivity and spectra resolution (e.g. reducing FWHW of resonance wavelength) to detect an analyte of a lower concentration.